The service life and reliability of a nitrogen die cylinder is directly related to the ability of the high pressure dynamic seal to maintain minimum leakage. The ability of the high pressure dynamic seal to maintain its proper function and operation has been found to be greatly affected by the amount and temperature of the lubricating oil film over which the seal travels. Surface contact wear and therefore leakage of the seal occurs rapidly when the seal is allowed to travel against a cylinder wall lacking an adequate film of lubricating oil.
Most nitrogen die cylinders only receive a film of lubricating oil on the cylinder walls during assembly. This can be supplemented by installing an oil soaked foam wiper in a groove next to the seal. However, with time, gravity tends to pull the oil film off of the cylinder. The seal itself, on each stroke, will push excess oil off the cylinder wall making the film even thinner after each stroke. The oil soaked film wiper can maintain the oil film only for a limited time before gravity and the wiping action of the seal displace the oil to areas in the nitrogen die cylinder where the oil no longer is of any benefit.
Others have suggested using the compressible working fluid (i.e., nitrogen) passing through an oil soaked sponge-like material to entrain oil for deposit on cylinder walls. See Wallis U.S. Pat. Nos. 4,005,763; 4,044,859; 4,076,103; and 4,342,448. However, in each case, the nitrogen die cylinder requires the oil to be entrained in the nitrogen working fluid. The Wallis systems do not rely on positive displacement or pressure differential to create a flow of lubricating oil. Moreover, the lubricating oil is only circulated in one direction, and thus, oil eventually collects in areas of the cylinder assembly where the oil is of no benefit.
Another approach is shown in Soman U.S. Pat. No. 2,815,254 which discloses an air cylinder having a lubricating means. However, the lubricating means relies on a direct pumping action and only delivers lubricating oil to one spot on the circumference of the seal. Soman does not disclose any working gas flow control device, nor does it permit the working gas to be mixed with the lubricating oil.
High temperatures can impair the lubricating properties of the oil film. A typical dynamic high pressure seal has a very small contact area against the cylinder wall. High gas pressure in die cylinders can cause very high unit loading on the seal and therefore generate high temperatures from friction during movement of the seal. A thin lubricating oil film is heated by this friction which can cause the oil to oxidize or break down. Since the seal slides over the same oil film on each stroke, it may not allow the oil sufficient time to dissipate heat.